The invention relates to intercom systems, and particularly, to transmission of low-frequency data in intercom systems.
In intercom systems, communication radios that operate in narrow bandwidths often use sub-audible tones or low-frequency tones for signaling information, and relatively higher frequency signals for voice information. The tones are generally retrieved from the voice signals (or audio signals) by means of filtering.
In an intercom system, a user wearing a belt pack (i.e., a portable radio unit) talks into an associated microphone, and pushes one or mores buttons of the belt pack, which causes different tones to be sent out along with the voice. When a base station receives the tones (and voice signals) from the belt pack, circuits in the base station decode the received signals. Depending on the decoded tone, the base station routes the corresponding voice signals to an audio channel associated with the decoded tone. In an intercom system having three channels (for example, an A channel, a B channel, and a stage announce channel), if a user wants to communicate with other users on the A channel, the user presses a button associated with the A channel thus generating a tone associated with the A channel. The belt pack worn by the user transmits the audio signals together with the tone signals from the user to the base station. The base station receives the tone and audio signals, decodes the tone signals, and directs the audio signal to the other users on the A channel.